Infrared cameras are used increasingly in a variety of operations such as law enforcement, facilities security, search and rescue, and national defense. Such applications all require detecting the presence of human beings, vehicles, and other objects in low-light situations where conventional binoculars or video cameras are otherwise impossible to use.
To provide compatibility with standard television-type raster scan displays, such cameras typically include a two-dimensional imaging system for receiving radiant energy. The imaging system usually includes a scanner, which is a device containing a number of reflectors to direct radiant energy from successive elemental areas of an input field of view to one or more radiant energy detectors. Although a single, gimballed reflector might be used to scan the desired field of view, two orthogonal reflectors are generally used, with each reflector being separately driven about a single axis.
For serial scanning one or both of the reflectors may be oscillated about an axis; or one axis may be scanned by using mechanisms such as a multifaceted polygonal mirror, sawtooth reflective wheel, or internally reflective carousel mounted on a high revolution-per-minute (RPM) motor.
However, there are several advantages to using a two-axis oscillating, or raster-scanning reflector arrangement as disclosed in U.S. Pat. No. 4,347,530 issued to Stetson on Aug. 31, 1982, and assigned to Inframetrics, Inc., the assignee of the present invention, the contents of which are hereby incorporated by reference. The technique described therein is to provide orthogonally-mounted horizontal and vertical reflectors, with each arranged to oscillate sinusoidally about a single axis. By using this technique, the mechanical complexity of high speed rotating assemblies is avoided, and the resulting mechanism is very light-weight and insensitive to shock and vibration. In addition, a high scan efficiency can be achieved, so that almost the entire scanned object field is available for presentation to the display.
Nonetheless, present imaging systems are not without their problems. Consider how a typical observer uses such a system. He or she first sets a desired field of view to be fairly wide, in order to detect activity in as large an object scene as possible. Upon noting an object of interest, the desired field of view will be narrower. In other words, the observer wishes to see the object with greater magnification, or otherwise cause objects of interest to appear larger in display.
With a two-axis scanner the field of view can be reduced almost instantaneously, by simply limiting the angle through which each of the reflectors travels while maintaining constant scanning frequency. This is not always convenient or aesthetic with a rotating multifaceted polygonal mirror arrangement, wherein the speed of rotation and thus the scanning frequency must be changed in order to change the resulting field of view. In particular, the momentum of the rotating mirror must also dissipate, and since this may take a minute or even longer to occur, the magnified image is clearly visible only after a waiting period.
However, two-axis scanners are also not without their drawbacks. Although the field of view can be changed almost instantaneously, the result is a picture in which the resolution has not changed. In other words, a magnified presentation of the object scene is provided to the display, but one in which the instantaneous field of view has not changed. Thus, elemental areas of the scene, or pixels, in the display appear to grow, and resolution is apparently lost, or at least, a high resolution display is not available.
A variable magnification optical telescope positioned at the input to the scanner can also be used to change the field of view. These typically require a mechanism of some sort to switch various magnifying lenses in and out of the input optical path to the scanner. However, this not only adds mechanical complexity to the entire system, but also results in further problems. In particular, the focal length of the systems different optical elements is then different with different fields of view. Thus, an object scene completely in-focus for one field of view will not necessarily be in focus for a different field of view. This then requires refocusing, or elaborate servomechanisms to compensate for the difference in focal length. The result is either difficulty in manually tracking objects, or an additional, expensive servomechanism.
What is needed is an imaging system which allows reducing the field of view with a simultaneous change in the instantaneous field of view, so that resolution of a magnified display increases accordingly.
In addition, an object in focus should also remain in focus when the field of view is changed, so that the observer does not lose track of objects which must be followed closely.
The system should be small, mechanically simple, easy to use, lightweight, and rugged, much as a set of optical binoculars, so that it can be carried about easily, and otherwise used in the field.
In the prior art, such as in U.S. Pat. No. 4,347,530 previously mentioned, two or more detectors were placed in the focal plane of the scanner so that two lines of the object scene could be scanned at a given time, or so that an improved detector signal to noise ratio could be obtained. However, there is no teaching or suggestion in the prior art that multiple detectors can be used to change the instantaneous field of view, or to provide a scanning system in which the required focus is not affected by changes in temperature or the desired field of view.